The world saw dramatic improvements in water availability during the 1980s, designated "International Water Supply and Sanitation Decade" by the UN. Nevertheless, reductions in diarrheal morbidity in much of the developing world have been modest, possibly because of a lack of sufficient parallel improvements in sanitation and hygiene. In a recent meta-analysis of 64 studies, Fewtrell and Colford (2004) found that water supply, water quality, hygiene, and sanitation programs all reduce diarrheal disease mortality and morbidity. However, multiple interventions did not reduce diarrheal disease any more than approaches that involved only one intervention. This suggests that a better fundamental understanding of the relationship between sanitation and diarrheal disease is needed to optimize future interventions, and also provides the background for this highly cross-disciplinary investigation of likely linkages between hydrogeology and the distribution of microbial pathogens in groundwater tapped by tubewells throughout southern Asia. Millions of rural households across Bangladesh, a country with a per capita GNP of SUS400, have devoted a total USS500M over the past 2 decades to the installation of private tube wells in the hope of improving their health. This was the response to efforts by international organizations, including UNICEF, the Bangladesh government, and many NGOs to reduce infant mortality by encouraging a switch from surface water to groundwater, which is generally less contaminated with human pathogens. The nearly universal switch of human consumption to groundwater in rural Bangladesh has been a disappointment for two main reasons: (1) at least a third of the population pumping groundwater from the 10 million existing tube wells in the country is exposed to toxic levels of arsenic (BGS/DPHE, 2001) and (2) diarrheal morbidity remains a severe problem for all age groups in rural Bangladesh. Indeed, diarrheal diseases are the largest cause of death among children under five. One possible explanation for the second disturbing finding is that poor sanitation in densely populated villages has resulted in the widespread microbial contamination of shallow aquifers. By building on a new understanding of how differences in local recharge of shallow aquifers are related to the geology of fluvio-deltaic aquifers, a knowledge that came out of on-going work focused on arsenic by Columbia University and partners in Bangladesh, this study seeks to explore, to our knowledge for the first time, whether spatial and temporal patterns in the distribution of certain forms of diarrheal disease are also influenced by local hydrogeology. Two recent findings motivate this study. The first is that the red paint applied on millions of private tubewells in Bangladesh containing >50 ug/L arsenic (the local standard, which is 5-fold higher than the WHO guideline of 10 ug/L) has led approximately one third of the exposed population to switch its consumption of untreated groundwater to a nearby well painted green because it contained <50 ug/L As (Ahmed, 2006). The second is the outcome of detailed field investigations in Columbia University's main study area of Araihazar showing that shallow (<20 m deep) tubewells that are low in As are typically located in areas where shallow sandy aquifers extend essentially to the surface, whereas shallow tubewells that are high in As are located in areas where shallow aquifers are capped by a low hydraulic conductivity layer of mud (i.e. clay or fine grained silt). Therefore, well testing for arsenic has re-directed a large proportion of the Bangladesh population towards drinking untreated groundwater from shallow aquifers that are relatively unprotected from the widespread microbial contamination of surface water bodies of the region due to poor sanitation. The concern is that, whereas well-switching had the undoubted benefit of reducing As exposure, it may also have had the unintended consequence of increasing human-exposure to microbial pathogens. Clearly, the goal of public health interventions in Bangladesh, and similarly affected countries, should be to reduce exposure to arsenic without increasing exposure to microbial pathogens. In support of this important goal, this team of investigators proposes to generate new knowledge concerning the ecology of organisms causing diarrheal disease in developing countries, by investigating the potential linkages between hydrogeology, the distribution of microbial pathogens in shallow aquifers and the occurrence of diarrheal disease in a large and well-studied population. We also aim to develop and test preliminary methods for predicting the occurrence of pathogen-contamination in aquifers based on occurrence of arsenic (which is already widely-monitored) and hydrogeologic/geomorphic setting (based on satellite imagery, field mapping and near-surface geophysics).